The Physics
Opus in profectus

Chemical Potential Energy

search icon



Calorimetry and so on.

Q = mcΔT

Burn a sample of the substance in oxygen, use the heat given off to warm a bit of water and watch the temperature rise.

When comparing substances, it's often more instructive to speak of their specific energy or specific work or gravimetric energy density or volumetric energy density (energy per mass) symbol e or w.

e =  E

or energy density (energy per volume) symbol u or η (eta) or ε (epsilon).

u =  E

rather than their energy.

simple compounds specific energy
fossil fuels specific energy
carbon (to CO2) 32.8 coal, anthracite > 32.6
carbon (to CO) 22.7 coal, bituminous 24.4–32.6
ethanol 29.7 coal, sub-bituminous 19.3–24.4
hydrogen 142 coal, lignite 14.7–19.3
methane 55.5 diesel 45.3
methanol 22.7 gasoline, automotive 45.8
nitromethane 11.6 gasoline, aviation 43.1
octane 47.9 jet a 43.3
propane 50.3 jet b 43.5
sulfur 9.16 kerosene 46.3
foods specific energy
oil, crude (petroleum) 41.9
oil, heating 42.5
carbohydrates 17.2 biomass fuels energy density
fats 38.9
proteins 17.2 alfalfa straw 18.4
explosives specific energy
charcoal 29.0
coconut husks 9.8
ANFO 6.3 coconut shells 17.9
BTTN   coffee husks 16.0
C4 6.3 cotton hulls 19.4
composition b   cotton stalks 17.4
composition c   dung, air dry 12.0
dynamite 7.5 groundnut shells 19.7
gunpowder 3.0 maize cobs 18.9
HMX   maize stalks 18.2
lead azide   peat 14.6
nitrocellulose   rice hulls 15.5
nitroglycerine 6.4 rice straw 15.2
PETN 5.8 sugar cane bagasse 19.0
RDX   sorghum bagasse 18.9
semtex   soybean stalks 19.4
silver azide   wheat straw 18.9
silver nitride 0.9 wood, green 10.9
sodium azide   wood, air dry 15.5
TATP   wood, oven dry 20.0
torpex 7.0    
TNT 4.7    
Specific energy of selected substances


A word about fuel based units.

fuel unit energy
cubic meter natural gas
(15 ℃, 1 atm)
37.1-38.7 Energy Information Administration
37.3 Carbon Dioxide Information Analysis Center
38.3 ConEdison (New York City)
37.9 Niagara Mohawk (New York State)
38.4 PSE&G (New Jersey)
ton of oil equivalent
(1,000 kg)
41,868 International Energy Agency
42,622 Statistics Norway
44,760 Carbon Dioxide Information Analysis Center
ton of coal equivalent
(1,000 kg)
29,308 International Energy Agency
29,310 Center for Energy Efficiency
29,290 Carbon Dioxide Information Analysis Center
ton of tnt 4,184 exact value by definition
Fuel-based energy units


Respiration (cellular respiration, that is, not pulmonary respiration or breathing as it is better known) is chemically identical to combustion, but it takes place at a much slower rate.

C6H12O6 + O2 → CO2 + H2O + energy

or in English

sugar combined with oxygen turns into carbon dioxide and water and releases energy

The energy content of foods is no longer measured directly much anymore. Instead, the food item in question is analyzed to determine the amount of protein, fat, and carbohydrate present in one serving. These numbers are then multiplied by the respective energy densities. In the US, where kilocalories are used instead of megajoules, the rule is 4-9-4: 4 kcal/g for protein, 9 kcal/g for fat, and 4 kcal/g for carbohydrate.

Energy density of macronutrients
foods kcal/kg (MJ/kg)
carbohydrates 4 17.2
fats 9 38.9
proteins 4 17.2

Adjustments are needed for indigestible fiber and specific energy factors/coefficient of digestibility.

Atwater system. Nutrition Labeling and Education Act of 1990.

Basal metabolic rate. Revised Harris-Benedict equations.

Qmen[kJ/day] =  0370 + 56.05 m[kg] + 20.08 h[cm] − 23.75 t[yr]
Qwomen[kJ/day] =  1872 + 38.69 m[kg] + 12.96 h[cm] − 18.12 t[yr]